1. Field of the Invention
The present invention relates to a device and an operating process for a gas quantity setting system, such as for the testing and/or certification of "low emission vehicles". In such systems a dilution air/exhaust gas mixture is taken for investigation. According to the present invention the system has a cross-sectionally variable, in particular continuously cross-sectionally variable, venturi nozzle with an axially adjustable, preferably conical, nozzle needle. The nozzle needle of the venturi nozzle is adjustable using a computing device, taking into consideration a temperature, a pressure and measured values of a mass rate of flow meter, in such a way that the dilution ratio (dilution factor) of the exhaust gas has a predetermined, preferably constant, value despite fluctuations in the feeding of quantities of exhaust gas.
2. Related Art
Gas quantity setting systems for the testing of vehicle exhaust gases are known, for example from U.S. Pat. No. 4,586,367 and from U.S. Pat. No. 3,603,155. These known systems operate with an average dilution rate, in order to determine the concentration of pollutants. The average dilution rate inevitably provides only an integral value overtime and it is not possible to ascertain an instantaneous result (for example in acceleration phases). Therefore, incorrect statements concerning the actual conditions are not ruled out. In fact, the conditions are changing constantly between exhaust gas and dilution air, since a driving cycle of the vehicle requires accelerations, decelerations etc., which leads to different exhaust gas/dilution air ratios.
German Utility Model G 90 14 528 discloses an exhaust gas quantity setting device which is more suitable when compared to the systems in the two U.S. patents referred to above. The integral evaluation is replaced by a direct evaluation. Thus, it becomes possible for the interrelationship of exhaust gas concentration and exhaust gas quantity with unique assignment to be modal, i.e., in predetermined sample quantities. The above-mentioned devices operate with known critical venturi nozzles, which set the desired total rate of flow quantities.